This invention relates to a film composition consisting of a rigid coating layer of high hardness, and, more particularly, it is concerned with the formation of a rigid coating layer from a coating material containing therein silica sol.
With the progress in the high polymer chemistry, various kinds of useful organic paint have been developed. On the other hand, inorganic paint, owing to its excellent refractoriness and heat-resistance as well as good applicability to an object to be painted, has also been used in many fields which require such inorganic paint. For the forming material of the inorganic coating film or layer, there have been used cement, lime, clay, alkali silicate, alkali phosphate, silica sol, and so forth. Of these various forming materials, silica sol is particularly superior in its heat-resistance and weather-resistance, hence its usefulness has been widely recognized. However, the paint containing therein the silica sol as the film forming material gives a coating film of low hardness, so that it can only be used in a limited field where the coating film does not need so high a hardness. To increase the hardness of the coating film to be formed from the paint containing therein the silica sol, there have been proposed various methods such as, for example, mixing a small amount of alkali silicate into it, or baking the coating film at a high temperature of 500.degree. C or above. These proposed methods, however, are still deficient in that, in the former method, the coating film undergoes a efflorescence and is discolored with lapse of time, and, in the latter method, a separate device to give the required high temperature for the baking is necessary, which reduces practicability of the method.